In the semiconductor industry, there has recently been a high-level of activity using strained Si-based heterostructures to achieve high carrier mobility structures for complementary metal oxide semiconductor (CMOS) applications. Traditionally, to boost performance of NFET and PFET devices, the prior art method to implement this has been to grow strained Si layers on thick (on the order of from about 1 to about 5 micrometers) relaxed SiGe buffer layers.
In order to produce relaxed SiGe material on a Si substrate, prior art methods typically grow, via epitaxy, a uniform, compositionally-graded or stepped SiGe layer to beyond the metastable critical thickness (i.e., the thickness beyond which dislocations form to relieve stress) on a single crystalline substrate, i.e., Si, and allow misfit dislocations to form, with the associated threading dislocations, through the SiGe buffer layer. Because the lattice parameter of SiGe is larger than that of Si, a compressive strain exists in the alloy layer as it is grown on Si.
When a typical prior art metastable strained SiGe layer is annealed at a sufficiently high temperature, misfit dislocations will form and grow thereby relieving the total strain on the film. In other words, the initial elastic strain of the film is relieved by the onset of plastic deformation of the crystal lattice. For the case of prior art metastable strained SiGe grown on a silicon-on-insulator (SOI) substrates, experiments have shown that under most annealing/oxidation conditions, the formation of misfit dislocations occurs early in the annealing history for temperatures greater than about 700° C.
Many of these defects are then either consumed or annihilated during the high-temperature annealing of the structure, however, the surface topography of the original misfit array persists during oxidation. Furthermore, SGOI substrate materials fabricated by thermal diffusion do not completely relax the SiGe alloy layer. Instead, the final SiGe lattice expands only to some fraction of the equilibrium value. If thick strained SiGe layers are grown on SOI substrates to allow greater relaxation during the subsequent thermal mixing procedure, then dislocations can be formed during the deposition of the film. Therefore, there is an upper limit to the thickness that the metastable SiGe layer can be formed on the SOI substrate without introducing defects in the as-grown material. If the deposited SiGe layer is unstrained (relaxed), then thick layers could be grown on SOI substrates without forming dislocations during growth.
In addition to growing thick SiGe layers atop an SOI substrate and then relaxing the SiGe layer by annealing/oxidation, it is also known to form SiGe-on-insulator substrates by wafer bonding and/or by oxygen implantation. These techniques for forming SGOI substrates require complicated and/or extra processing steps, which add additional time and/or cost to the production of the SGOI material.
Despite the above techniques for forming SGOI substrate materials, there is still a need for providing another method that is capable of forming highly-relaxed SiGe alloy layers over an insulating layer.